In the manufacture of paper an aqueous cellulosic suspension or slurry is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1 percent, and often below 0.5 percent ahead of the paper machine, while the finished sheet must have less the 6 weight percent water. Hence the dewatering aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
The dewatering method of the least cost in the process is drainage, and thereafter more expensive methods are used, for instance vacuum, pressing, felt blanket blotting and pressing, evaporation and the like, and in practice a combination of such methods are employed to dewater, or dry, the sheet to the desired water content. Since drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and hence improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of the manufacture is retention of furnish components on and within the fiber mat being formed during papermaking. A papermaking furnish contains generally particles that range in size from about the 2 to 3 millimeter size of cellulosic fibers, to fillers at a few microns, and to colloids. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would in significant portion pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed during papermaking.
One method of improving the retention of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the use of a coagulant/flocculant system, added ahead of the paper machine. In such a system there is first added a coagulant, for instance a low molecular weight cationic synthetic polymer or a cationic starch to the furnish, which coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly cellulosic fines and mineral fillers, and thereby accomplishes a degree of agglomeration of such particles, followed by the addition of a flocculant. Such flocculant generally is a high molecular weight anionic synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into large agglomerates. The presence of such large agglomerates in the furnish as the fiber mat of the paper sheet is being formed increases retention. The agglomerates are filtered out of the water onto the fiber web, where unagglomerated particles would to a great extent pass through such paper web.
While a flocculated agglomerate generally does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained an amount of gelatinous material, when such flocs are filtered by the fiber web the pores thereof are to a degree reduced, reducing the drainage efficiency therefrom. Hence the retention is being increased with some degree of deleterious effect on the drainage.
Another system employed to provide an improved combination of retention and dewatering is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, inventors Langley et al., issued respectively June 28, 1988 and Apr. 3, 1990, incorporated hereinto by reference. In brief, such method adds to the aqueous cellulosic papermaking suspension first a high molecular weight linear cationic polymer before shearing the suspension, followed by the addition of bentonite after shearing. The shearing generally is provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shearing breaks down the large flocs formed by the high molecular weight polymer into microflocs, and further agglomeration then ensues with the addition of the bentonite clay particles.
Another system uses the combination of cationic starch followed by colloidal silica to increase the amount of material retained on the web by the method of charge neutralization and adsorption of smaller agglomerates. This system is described in U.S. Pat. No. 4,388,150, inventors Sunden et all, issued June 14, 1983.
Dewatering generally, and particularly dewatering by drainage, is believed improved when the pores of the paper web are less plugged, and it is believed that retention by adsorption in comparison to retention by filtration reduces such pore plugging.
Greater retention of fines and fillers permits, for a given grade of paper, a reduction in the cellulosic fiber content of such paper. As pulps of less quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes even more important because the fines content of such lower quality pulps is greater generally than that of pulps of higher quality.
Greater retention of fines, fillers and other slurry components reduces the amount of such substances lost to the white water and hence reduces the amount of material wastes, the cost of waste disposal and the adverse environmental effects therefrom.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation is determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90 percent, the formation parameter generally abruptly declines from good formation to poor formation. It is at least theoretically believed that as the retention mechanisms of a given papermaking process shift from filtration to adsorption, the deleterious effect on formation, as high retention levels are achieved, will diminish, and a good combination of high retention with good formation is attributed to the use of bentonite in U.S. Pat. No. 4,913,775.
It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
It is also desirable to use additives that can be delivered to the paper machine without undue problems. An additive that is difficult to dissolve, slurry or otherwise disperse in the aqueous medium may require expensive equipment to feed it to the paper machine. When difficulties in delivery to the paper machine are encountered, the additive is often maintained in aqueous slurry form by virtue of high energy imput equipment. In contrast, additives that are easily dissolved or dispersed in water require less energy and expense and their uniformity of feed is more reliable.